United Kingdom · GBP
Economy 7 vs Single-Rate Cost Calculator
Compare the same household electricity use on a single-rate plan and Economy 7 or another compatible time-of-use plan. Include each plan’s own standing charge and see how your night-time energy share affects the result.
Compare your two electricity plans
Use the same billing days and total electricity imported from the grid for both plans. Enter each plan's prices and daily charge, including an explicit 0 where no daily charge applies.
Enter 30 for £0.30/kWh. Use prices with applicable tax included. Zero and negative unit prices are supported; each entered price stays constant for this comparison.
Enter each daily charge separately in pence, including applicable tax. It is charged once per day for the connection.
Single-rate plan
Energy + daily charges for your period
- Energy charge
- Daily charges
Time-of-use plan
Energy + daily charges for your period
- Energy charge
- Daily charges
- Total grid import
- Day / peak share
- Night / off-peak share
- Third-period share
- Average time-of-use energy price
The average energy price excludes daily charges. Shares are undefined when total import is zero. Displayed values are rounded; comparisons use unrounded inputs.
Enter your billing days, energy and both plans to compare their charges.
Enter valid prices and an energy split to find a break-even share.
Calculation and comparison limits
Single/flat-rate total = total imported kWh × its unit price ÷ 100 + billing days × its daily charge ÷ 100. Time-of-use total = (peak kWh × peak price + off-peak kWh × off-peak price + any third-period kWh × its price) ÷ 100 + billing days × its daily charge ÷ 100.
For break-even, K is total kWh, D is days and f is the third-period fraction, or 0 with two periods. The off-peak fraction of total energy at equality is [K × (peak price × (1 − f) + third price × f − flat price) + D × (time-of-use daily charge − flat daily charge)] ÷ [K × (peak price − off-peak price)]. Prices and charges use the same pence or cents units. Keep the third-period share fixed and move energy only between peak and off-peak. The possible off-peak range is 0 to (1 − f). Equal peak/off-peak prices and zero imported energy need separate treatment.
This compares the entered energy and daily charges only, for the entered period. It does not forecast annual use, infer electricity from clock hours, choose a retailer or establish eligibility. Dynamic interval settlement, solar export credits, demand charges, block/tier prices, controlled-load circuits, add-ons, discounts and other fees are excluded. Check the actual tariff's time windows and terms before relying on a scenario. Your entries remain in this tab and are not saved or sent.
Use the same household energy and billing days
Start with imported electricity over one clearly defined period. For cumulative meter registers, use the end reading minus the start reading for each matching register. Confirm which register belongs to each price band. Each kWh must appear once, and all readings must cover the same elapsed days.
The flat or single-rate plan prices the sum of that energy. The time-of-use plan prices the amount assigned to each of its own periods. If you are considering a tariff with different time windows, your existing bill's band totals may not match: use suitable interval data or label the new split as an estimate. A share is a percentage of total kWh, not of hours, appliances or the cost.
Use the whole household's relevant grid import when comparing a whole-house tariff. An EV charging session or a heater reading alone does not capture a price change affecting the rest of the home. Electricity used directly from solar and electricity exported to the grid are not imported energy. Do not subtract exported kWh from the imported kWh entered here.
Check the day/night arrangement on your tariff
Citizens Advice explains that Economy 7 normally offers seven off-peak hours, while other time-of-use tariffs can have different structures. Check your supplier's actual hours, including clock changes and any split periods. Seven cheap hours do not imply that seven twenty-fourths of your kWh receive that price.
Whole-house time bands and restricted heating supplies are different arrangements. OVO's one-meter, two-rate guidance describes both. A cheaper price for an eligible heating circuit or EV charging credit must not be used automatically for all household night-time import. Ask the supplier which energy and charges the proposed plan covers.
Enter each plan's own standing charge in pence per day. A daily charge is added once for each billing day, even when imported kWh is zero. Use prices with applicable tax included on the same basis; do not add tax again to a tax-inclusive quote. If prices change during the period, calculate the affected portions separately.
A worked comparison with illustrative prices
These figures demonstrate the arithmetic. They are invented scenarios, not current tariffs, recommended plans, typical household use or a forecast. The example stays fixed when you edit the calculator.
Suppose the household imports 1,000 kWh over 30 days. A single-rate plan charging 30p/kWh and 50p/day costs £315: £300 for energy plus £15 in standing charges.
On a two-rate plan charging 40p/kWh by day, 20p/kWh at night and 60p/day, a 50% night share means 500 day kWh and 500 night kWh. The total is £318: £300 for energy plus £18 in standing charges. The single-rate plan costs £3 less at this split.
The equal-cost night share is 51.5% of total imported kWh. At 515 night kWh and 485 day kWh, the two-rate energy charge is £297; adding £18 gives £315. Above that share the two-rate plan costs less under these assumptions; at 51.5% the plans tie.
Read the break-even result within its limits
The break-even calculation holds total imported kWh, billing days, prices and daily charges constant. With three periods, it also holds the third-period share constant. Only energy assigned to peak/day and off-peak/night changes. Moving several devices may not achieve that split, and a short meter sample may not represent a heating season or a year.
A cheaper off-peak rate can produce a minimum off-peak share; reversed prices reverse which side costs less. Equal prices may make the split irrelevant. A boundary can be a tie, or there may be no equal-cost share in the possible range. With zero imported kWh, only daily charges can be compared.
This tool uses two or three entered energy prices held constant for the period. It does not settle dynamic interval prices or calculate demand charges, export credits, controlled-load circuits, usage tiers, discounts, subscriptions, exit fees or meter-change fees. Check these items and tariff eligibility before deciding to switch. For a single appliance at one usage price, use the UK electricity cost calculator.
Common questions
Is Economy 7 cheaper than a single-rate tariff?
It depends on both plans’ unit prices and standing charges, your total imported electricity, and how much of that energy falls in each time period. A cheaper night rate can be offset by a higher day rate or standing charge. Compare the same total household kWh and billing days; the result here covers entered energy and daily charges only.
What percentage of electricity must I use at night to break even?
There is no universal percentage. The calculator finds the equal-cost night or off-peak share from your entered prices, daily charges, days and total kWh. With a third period, its share stays fixed while energy moves between day and night. A threshold can fall outside the possible range, and equality is a tie rather than a saving.
What hours are off-peak on Economy 7?
Economy 7 normally provides seven off-peak hours in 24 hours, but the actual times depend on the supplier, tariff and meter arrangement and can change with the clocks. Check your supplier’s timetable and register mapping. Seven hours does not mean that 7/24 of your electricity is off-peak; use the kWh recorded in those periods.
Does an Economy 7 night rate apply to the whole house?
Some Economy 7 arrangements charge all household electricity at the rate applying at that time. Other products restrict cheaper electricity to eligible heating circuits or apply credits to eligible EV charging. Confirm your exact arrangement; this whole-house comparison does not model separate controlled circuits, EV-only credits or multiple-meter charges.
Can I use this calculator in Northern Ireland?
The arithmetic can compare compatible tariffs using your own unit prices, daily charges and imported kWh. No Great Britain price-cap rate is supplied. The sources here do not establish availability or eligibility for a particular Northern Ireland tariff; check those details with the supplier.
Will this show my full bill or tell me whether I can switch?
No. It compares imported energy and the daily charges you enter over the same period. It excludes gas, exit or meter-change fees, discounts, credits, exports, dynamic interval prices and other unmodelled charges. Confirm meter compatibility, tariff eligibility and contractual terms with the supplier before switching.
More UK calculators
Sources and checking your inputs
- Ofgem: energy price cap and standing charges explained — Unit prices and daily standing charges are distinct; a standing charge remains payable when no energy is used. No Ofgem price is used as an input here.
- Citizens Advice: choosing your energy tariff — Explains single-rate, time-of-use and dynamic tariff structures. This advice is for England, with links to other national versions.
- Citizens Advice: electricity use and off-peak hours — Check the actual supplier timetable, tariff and meter readings; hours can change with the clocks.
- OVO: one meter with two rates — Distinguishes its whole-house Economy 7 arrangement from products restricted to specified heating loads. Its product details are not assumed for other suppliers.
- Octopus Energy: Economy 7 meters and tariffs — Explains separate meter registers and supplier-specific time arrangements. Its suitability shorthand is not used as a universal break-even rule.
- Ofgem: switching your home energy supplier — Check meter compatibility, terms and relevant fees separately from this cost calculation.
Source review: 3 October 2026. These sources support tariff definitions and input guidance; all prices and energy in the calculator come from your entries. The worked examples are illustrative. Report a correction.